Catalysts For Soot Combustion Research Articles

Promoting diesel soot combustion efficiency by tailoring the shapes and crystal facets of nanoscale Mn3O4

Various shapes (hexagonal nanoplate, octahedral, and nanoparticle) of nanoscale Mn3O4 have been successfully prepared via facile hydrothermal and co-precipitation methods, respectively, and evaluated for their performance on diesel soot combustion. The catalytic performance of hexagonal nanoplates Mn3O4 (Mn3O4-HNS) is distinguished from the samples with other shapes by its superior activity on soot combustion with Tm of 407.7 °C and SmCO2 of 99.1% at the gas hourly space velocity of 9990 h−1 with the feed composition of 2500 ppm NO/5 vol.%O2/N2 under loose contact mode. The physicochemical properties of Mn3O4 were systematically examined by different characterization techniques, including XRD, FTIR, BET, H2-TPR, FE-SEM, HR-TEM, XPS, soot-TPR, and NO/NO + O2-TPSR. The different crystal shapes controlled by the preparation method are found to influence the extent of exposed Mn3O4 crystal facet, as determined by HR-TEM, which, in turn, was found to correlate with the catalytic performance. The kinetic study of soot combustion over these samples further revealed that the Ea obtained from three samples ranked as the order of (112) < (101) < (220) facets in reverse order of activity data, confirming the crystal-facet dependent reactivity. In view of the difference in catalytic activity and their physicochemical properties as estimated by various techniques, the superior catalytic performance of Mn3O4-HNS for diesel soot combustion was originated from its exposed (112) facet in association with its good lower temperature reducibility, abundant surface Mn4+ and active oxygen species, and the enhanced NO oxidation capability. Furthermore, the best performing Mn3O4-HNS displayed the good stability through recycling test. Our work may provide an important insight on the design strategies to develop the high-efficient soot combustion catalysts under the working conditions of the diesel engines.

Glass catalysts for soot combustion and methods of manufacturing the same

Glass catalysts for soot combustion and methods of manufacturing the same

Effect of Dopant Loading on the Structural and Catalytic Properties of Mn-Doped SrTiO3 Catalysts for Catalytic Soot Combustion

Soot particles have been associated with respiratory diseases and cancer. To decrease these emissions, perovskite-mixed oxides have been proposed due to their thermal stability and redox surface properties. In this work, SrTiO3 doped with different amounts of Mn were synthesized by the hydrothermal method and tested for soot combustion. Results show that at low Mn content, structural distortion, and higher Oads/Olat ratio were observed which was attributed to the high content of Mn3+ in Ti sites. On the other hand, increasing the Mn content led to surface segregation of manganese oxide. All synthesized catalysts showed mesopores in the range of 32–47 nm. In the catalytic combustion of soot, the samples synthesized in this work lowered the combustion temperature by more than 100 °C compared with the uncatalyzed reaction. The sample doped with 1 wt % of Mn showed the best catalytic activity. The activation energy of these samples was also calculated, and the order of decreasing activation energy is as follows: uncatalyzed &gt; Mn0 &gt; Mn8 &gt; Mn4 &gt; Mn1. The best catalytic activity for Mn1 was attributed to its physicochemical properties and the mobility of the oxygen from the bulk to the surface at temperatures higher than 500 °C.

SnO2 promoted by alkali metal oxides for soot combustion: The effects of surface oxygen mobility and abundance on the activity

In this study, SnO2-based catalysts promoted by different alkali metal oxides with a Sn/M (M = Li, Na, K, Cs) molar ratio of 9/1 have been prepared for soot combustion. In comparison with the un-modified SnO2 support, the activity of the modified catalysts has been evidently enhanced, following the sequence of CsSn1-9 > KSn1-9 > NaSn1-9 > LiSn1-9 > SnO2. As testified by Raman, H2-TPR, soot-TPR-MS, XPS and O2-TPD results, the incorporation of various alkali metal oxides can induce the formation of more abundant and mobile oxygen species on the surface of the catalysts. Moreover, quantified results have proved that the amount of the surface active oxygen species is nearly proportional to the activity of the catalysts. CsSn1-9, the catalyst promoted by cesium oxide, owns the largest amount of surface mobile oxygen species, thus having the highest activity among all the studied catalysts. It is concluded that the amount of surface active and mobile oxygen species is the major factor determining the activity of the catalysts for soot combustion.

Plausibility of potassium ion-exchanged ZSM-5 as soot combustion catalysts

Potassium (K) ion-exchanged ZSM-5 zeolites were investigated for catalytic soot combustion. X-ray absorption fine-structure (XAFS), Raman, in situ IR and NH3-temperature programmed desorption (NH3-TPD) confirmed the location of K+ at the ion-exchanged sites. Temperature-programmed oxidation (TPO) reactions showed that K-ZSM-5 decreased ignition tempeatures of soot combustion and increased selectivity to CO2. The improved activity for soot combustion by increasing K+-exchanged amounts via decreasing the Si/Al ratio reinforced the K+ ions participating in soot combustion. 18O2 isotopic isothermal reactions suggested the activation of gaseous oxygen by the K+ ions. This demonstrated a new appliction of alkali metal exchanged zeolites and the strategy for enhancement of catalytic soot combustion activity.

The Effect of Sr Addition in Cu- and Fe-Modified CeO2 and ZrO2 Soot Combustion Catalysts

This study investigates the activity of transition and alkaline-earth metal-doped catalysts supported on ceria or zirconia for the NOx-assisted oxidation of diesel particulate. A series of Cu- and Fe-impregnated catalysts over CeO2 and ZrO2 supports were prepared by incipient wetness impregnation and characterized by BET, X-ray diffraction (XRD), and temperature-programmed reduction (TPR) experiments while their catalytic activity was investigated in NOx-assisted reaction by means of temperature programmed oxidation experiments (TPO). Higher activity was achieved by copper modified catalysts; the addition of Sr positively affected the performance of the materials, suggesting a synergic effect between transition metals and alkaline-earth metal. The role of copper is correlated to the oxidation of NO to NO2, while strontium seems to be mainly involved in the storage of NOx species.

Crystal facet-dependent reactivity of α-Mn2O3 microcrystalline catalyst for soot combustion

In this work, a series of α-Mn2O3 micro crystals with different morphologies and crystal shapes were successfully prepared by hydrothermal method and has been investigated as potential soot combustion catalysts for the first time. The activity data showed that the soot combustion efficiency were markedly affected by the shape of the prepared α-Mn2O3 catalysts, among which their activity ranked as α-Mn2O3-cubic>α-Mn2O3-truncated octahedral>α-Mn2O3-octahedra. As revealed by various physicochemical characterization techniques such as XRD, SEM, HR-TEM, FT-IR, BET, H2-TPR, O2-TPD, NO- and NO+O2-TPSR, the enhanced activity as well as selectivity over α-Mn2O3-cubic is originated with the nature of the exposed α-Mn2O3 (001) surface facets, on which the higher concentration of low-coordinated surface oxygen sites facilitates the oxygen activation and improves surface redox properties, thereby accelerating the formation of the crucial intermediate i.e., NO2 formation by NO oxidation and thus promoting the overall soot combustion efficiency. Moreover, the kinetic study performed under isothermal condition provided solid evidence and proof to support that it is the exposed crystal facet, rather than surface area, to be critical to determine the catalytic performance of soot combustion. Under loose contact condition, soot combustion efficiency over best performing α-Mn2O3-cubic catalyst was further enhanced in the presence of water, CO and hydrocarbons, which are essential components in real diesel exhaust. Furthermore, the cubic α-Mn2O3 displayed excellent durability against structural collapse upon repeated recycling test and accelerated deactivation test, demonstrating its promise for the practical use in diesel soot combustion.

Potassium‐Activated Wire Mesh: A Stable Monolithic Catalyst for Diesel Soot Combustion

AbstractPotassium‐modified FeCrAl alloy wire mesh was developed as a catalytic diesel particulate filter to suppress the emission of soot from a diesel engine. Potassium species were deposited on wire mesh by a chemical vapor deposition method, in which a model soot was used to convert KOH into metallic K at high temperatures to subsequently activate the wire mesh. Tests showed that metallic K reacted with the enriched Al2O3 component on the surface derived from segregation and successive oxidation during precalcination. The resulting layer of K‐O‐Al species offers remarkable activity and stability for the catalytic oxidation of diesel soot. The K‐activated wire mesh could lower the initial temperature of soot combustion and maintain the activity for several cycles.

Ceria‐based nanomaterials as catalysts for CO oxidation and soot combustion: Effect of Zr‐Pr doping and structural properties on the catalytic activity

In this work, we investigated a set of ceria‐based catalysts prepared by the hydrothermal and solution combustion synthesis. For the first time to our knowledge, we synthesized nanocubes of ceria doped with zirconium and praseodymium. The catalysts were tested for the CO and soot oxidation reactions. These materials exhibited different surface reducibility, as measured by H2‐TPR, CO‐TPR and Soot‐TPR, despite their comparable chemical compositions. As a whole, Soot‐TPR appears a suitable characterization technique for the soot oxidation catalysts, whereas CO‐TPR technique allows to better discriminate among the CO oxidation activities. Praseodymium contributes positively toward the soot oxidation. On the other hand, it has an adverse effect on the CO oxidation over the same catalysts, as compared to pure ceria. The incorporation of zirconium into the ceria lattice does not have a direct beneficial effect on the soot oxidation activity, although it increases the catalyst performances for CO oxidation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 216–225, 2017

Effects of Au–Ce strong interactions on catalytic activity of Au/CeO2/3DOM Al2O3 catalyst for soot combustion under loose contact conditions

Au/3DOM (three-dimensionally ordered macroporous) Al2O3 and Au/CeO2/3DOM Al2O3 were prepared using a reduction-deposition method and characterized using scanning electron microscopy, N2 adsorption-desorption, X-ray diffraction, transmission electron microscopy, ultraviolet-visible spectroscopy, temperature-programmed hydrogen reduction, and X-ray photoelectron spectroscopy. Au nanoparticles of similar sizes were well dispersed and supported on the inner walls of uniform macropores. The norminal Au loading is 2%. Al–Ce–O solid solution in CeO2/3DOM Al2O3 catalysts can be formed due to the incorporation of Al3+ ions into the ceria lattice, which causes the creation of extrinsic oxygen vacancies. The extrinsic oxygen vacancies improved the oxygen-transport properties. The strong metal–support interactions between Au and CeO2 increased the amount of active oxygen on the Au nanoparticle surfaces, and this promoted soot oxidation. The activities of the Au-based catalysts were higher than those of the supports (Al2O3 or CeO2/3DOM Al2O3) at low temperature. Au/CeO2/3DOM Al2O3 had the highest catalytic activity for soot combustion, with T10, T50, and T90 values of 273, 364, and 412 °C, respectively.

Effect of A-site deficiency in LaMn0.9Co0.1O3 perovskites on their catalytic performance for soot combustion

The influence of lanthanum stoichiometry in Ag-doped (La1-xAgxMn0.9Co0.1O3) and A-site deficient (La1-xMn0.9Co0.1O3-δ) perovskites with x equal to 10, 20 and 30 at.% has been investigated in catalysts for soot combustion. The catalysts were prepared by the amorphous citrate method and characterized by XRD, nitrogen adsorption, XPS, O2-TPD and TPR. The formation of a rhombohedral excess-oxygen perovskite for Ag-doped and a cubic perovskite structure for an A-site deficient series is confirmed. The efficient catalytic performance of the larger Ag-doped perovskite structure is attributed to the rhombohedral crystalline structure, Ag2O segregated phases and the redox pair Mn4+/Mn3+. A poor catalytic activity for soot combustion was observed with A-site deficient perovskites, despite the increase in the redox pair Mn4+/Mn3+, which is attributed to the cubic crystalline structure.

Catalytic oxidation of diesel soot particulates over Ag/LaCoO3 perovskite oxides in air and NOx

Ag/LaCoO3 perovskite catalysts for soot combustion were prepared by the impregnation method. The structure and physicochemical properties of the catalysts were characterized using X-ray diffraction, N2 adsorption-desorption, H2 temperature-programmed reduction, soot temperature- programmed reduction, and X-ray photoelectron spectroscopy. The catalytic activity of the catalysts for soot oxidation was tested by temperature-programmed oxidation in air and in a NOx atmosphere. Metallic Ag particles were the main Ag species. Part of the Ag migrated from the surface to the lattice of the LaCoO3 perovskite, to form La1-xAgxCoO3. This increased the amount of oxygen vacancies in the perovskite structure during thermal treatment. Compared with unmodified LaCoO3, the maximum soot oxidation rate temperature (Tp) decreased by 50–70 °C in air when LaCoO3 was partially modified by Ag, depending on the thermal treatment temperature. The Tp of the Ag/LaCoO3 catalyst calcined at 400 °C in a NOx atmosphere decreased to about 140 °C, compared with that of LaCoO3. Ag particles and oxygen vacancies in the catalysts contributed to their high catalytic activity for soot oxidation. The stable catalytic activity of the Ag/LaCoO3 catalyst calcined at 700 °C in a NOx atmosphere was related to its stable structure.

Synergic effect of Cu/Ce0.5Pr0.5O2-δ and Ce0.5Pr0.5O2-δ in soot combustion

A series of 5%Cu/Ce0.5Pr0.5O2-δ and Ce0.5Pr0.5O2-δ mixed oxides have been prepared and combined in different ratios. The resulting catalysts have been characterized by N2 adsorption, XRD, Raman spectroscopy and H2-TPR and tested for soot combustion by means of temperature-programmed experiments. The optimum catalyst for soot combustion in NOx/O2/N2 atmosphere is the mixture containing 40% Cu/Ce0.5Pr0.5O2-δ and 60% Ce0.5Pr0.5O2-δ. This mixture is more active than a reference catalyst containing the same amount of copper distributed in the whole Ce-Pr mixed oxide support. The benefit of mixing Ce0.5Pr0.5O2-δ particles with and without copper in a single catalyst formulation is that the participation of the two soot combustion mechanisms based on active oxygen and NO2, respectively, is optimized. The particles with copper mainly promote the catalytic oxidation of NO to NO2 (the NOx-assisted mechanism) while those without copper are more effective in promoting the active oxygen mechanism. If copper is loaded homogeneously in all the Ce0.5Pr0.5O2-δ particles, the positive effect of copper improving NO2 production is offset by the lower efficiency of the active oxygen mechanism, due to a lack of active oxygen on the Ce0.5Pr0.5O2-δ support.

An oxygen pool from YBaCo4O7-based oxides for soot combustion

Soot, often referred to as black carbon emitted from diesel engines, is not only a particulate matter pollutant but also a light-absorbing agent that may affect global climate, but can be effectively controlled using a catalytic diesel particulate filter (DPF). A new YBaCo4O7+δ-type oxygen storage material is reported as an effective catalyst for soot combustion. Isotopic isothermal reactions demonstrate the activation of gaseous oxygen and subsequent oxygen storage and reaction/desorption during an oxidation process. High activity and structural stability are achieved by the substitution of Co with Al and Ga to form YBa(Co0.85Al0.075Ga0.075)4O7+δ. The specific rates at 300 °C of YBaCo4O7+δ and YBa(Co0.85Al0.075Ga0.075)4O7+δ, normalized by surface areas, are an order of magnitude higher than those of CeO2-based oxides. This kind of oxygen-storage material acts as an oxygen pool, which ensures that the accumulated soot on a DPF can be promptly combusted.

Potential of Cu–saponite catalysts for soot combustion

A variety of H– and Na–saponite supports have been prepared by several synthesis approaches; the best activity for soot combustion was achieved with copper oxide supported on a Na–saponite prepared at pH 13 and with surfactant.

Nanostructured ceria-praseodymia catalysts for diesel soot combustion

Nanostructured ceria-praseodymia catalysts with different praseodymium contents have been prepared through hydrothermal synthesis to study the effect of Pr as a dopant and the effect of morphology towards soot combustion under “loose” and “tight” soot-catalyst conditions. Samples synthesized through solution combustion synthesis (SCS) have also been prepared as comparative materials. Studies in physicochemical properties of the catalysts have been carried out using complementary techniques. The present work also resorts to soot-TPR as an unconventional method of investigating the ability of solid catalysts to initiate soot oxidation in the absence of bulk oxygen. Ce50Pr50 catalyst (where 50 indicates the atomic percentage of cerium as well as of praseodymium) with mixed structures of nanorods and nanocubes has attained the best catalytic performances, thanks to the high lattice oxygen mobility and the easy reducibility. The insertion of Pr cations to the ceria framework enhances the number of redox sites on the surface, thus generating more oxygen vacancies. As a whole, activity tests in general have proven that despite having relatively low surface areas, ceria-praseodymia nanocubes and nanorods facilitated soot combustion reaction more actively than SCS-based ceria-praseodymia catalysts with larger surface areas. This evidences the beneficial effect of well-defined nanostructures in soot combustion, due to their possession of highly reactive low-index facets (100) and (110). Within SCS-based samples, however, the specific surface area overshadows the importance of praseodymium. This eventually marks the synergistic combination of well-defined nanostructures and praseodymium as a dopant.

K-supported catalysts for diesel soot combustion: Making a balance between activity and stability

Two typical K-supported oxides (K/Al2O3 and K/TiO2) were selected as catalysts for diesel soot combustion in order to evaluate the relationship between activity and stability. The surface species were first characterized by X-ray powder diffraction (XRD), X-ray absorption fine-structure (XAFS), infrared analysis (IR), X-ray photoelectron spectroscopy (XPS), in situ IR of CO2 adsorption and temperature-programmed desorption of CO2 (CO2-TPD). Then the activity was studied by temperature-programmed oxidation (TPO), temperature-programmed reduction with soot (Soot-TPR) and in situ IR of soot combustion reactions. Lastly, the stability of the catalyst was checked by washing with water and analysis for K. This showed that K/TiO2 displayed negligible activity due to the immobilization of K+ ions in TiO2 to form K2Ti6O13. However, K/Al2O3 exhibited superior activity owing to the presence of active free K+ ions on the surface. The downside is the leaching of K+ ions into the effluent containing some water vapor, resulting in its poor stability – the other side of the coin for the highly active free K species. One of the suggested strategies for making a balance between activity and stability is encapsulating active K species in the channel of the catalyst as demonstrated by the tunneled cryptomelane.

Investigations into nanostructured ceria–zirconia catalysts for soot combustion

A set of nanostructured ceria–zirconia catalysts with different Zr-contents and structural features has been prepared to study the effect of both the Zr-amount and its surface-dependent activity towards soot combustion under different experimental conditions (namely in “loose” and “tight” soot-catalyst contact). A ceria–zirconia sample has been synthesized by means of solution combustion synthesis (SCS) for comparison purposes. The physico-chemical properties of the catalysts have been investigated using complementary techniques.The best catalytic performances have been achieved for the Ce0.9Zr0.1O2–NP catalyst (where NP means nano-polyhedra and 0.9 indicates the atomic ratio of Ce/Ce+Zr), due to the higher mobility of the lattice oxygen within the solid, and its easier reducibility, compared to high-surface area catalysts with the same Ce/Zr ratio. Moreover, better activities, in terms of soot conversions, have been reached for Ce0.9Zr0.1O2–NP, than similar nano-polyhedra with higher Zr-amounts (denoted as CexZr1−xO2–NP, where x=0.8 or 0.7). The substitution of some Ce4+ for Zr4+ ions favors the formation of defects (i.e., oxygen vacancies) in the ceria lattice, thus inducing a distortion of the oxygen sublattice. However, the amount of redox Ce species decreases as the Zr-content increases. It therefore seems that the incorporation of Zr4+ into the ceria lattice does not have a direct beneficial effect on the oxidation activity for catalysts calcined at low/mild temperatures, since it decreases the population of surface redox-active centers, which depend directly on the surface density of the Ce3+–Ce4+ species. On the other hand, lower soot conversion values have been reached for both mesoporous (Ce0.9Zr0.1O2–M) and microporous (50-Ce0.9Zr0.1O2–NP/FAU) samples with the same Ce/Zr ratio. Moreover, the comparative catalyst (Ce0.9Zr0.1O2–SCS) exhibited worse activities than the porous materials, thus confirming the key role of the textural properties for this oxidation reaction.

The role of crystallite size of iron oxide catalyst for soot combustion

A series hematite structure iron oxide nanoparticles of various sizes was synthesized and tested in the soot combustion in oxygen rich conditions in the tight and loose contact modes. The catalysts were characterized by XRD, TEM, and Raman Spectroscopy, whereas the catalytic reactivity with model soot (Printex U) was evaluated by TGA/DTA method. It was found that the catalysts activity is size dependent for the particles dimensions in the range of 5–100nm with the smallest one exhibiting the highest activity (lowest soot ignition temperature). The most active nanohematite particles, formed in situ in the laboratory and real process conditions of light fuel oil combustion with addition of fuel borne catalyst (FBC) precursors, exhibit the beneficial size range of 5–15nm.

Active oxygen by Ce–Pr mixed oxide nanoparticles outperform diesel soot combustion Pt catalysts

A Ce0.5Pr0.5O2 mixed oxide has been prepared with the highest surface area and smallest particle size ever reported (125m2/g and 7nm, respectively), also being the most active diesel soot combustion catalyst ever tested under realistic conditions if catalysts forming highly volatile species are ruled out. This Ce–Pr mixed oxide is even more active than a reference platinum-based commercial catalyst. This study provides an example of the efficient participation of oxygen species released by a ceria catalyst in a heterogeneous catalysis reaction where both the catalyst and one of the reactants (soot) are solids. It has been concluded that both the ceria-based catalyst composition (nature and amount of dopant) and the particle size play key roles in the combustion of soot through the active oxygen-based mechanism. The composition determines the production of active oxygen and the particle size the transfer of such active oxygen species from catalyst to soot.